I have used in a similar way as Hud as an additive to run of the mill alkyd paints like Jotun Pilot I & Pilot II. Worked quite well making it easier to brush and keeping a wet edge, especially in an Aussie summer.
Interestingly it had no effect when added to a single
enamel like Killrust super epoxy
enamel. When I called the Flood Company for info, they confirmed my view that whatever Killrust put in that paint, it "killed" off any advantage of the Penetrol. Made me think they were quite honest and up front about their product.